1. Field of the Invention
The present invention relates to a DC motor driver for use in a driving source for an image forming device or the like, and more particularly to avoidance of braking action when switching phases of the motor.
2. Description of the Related Art
FIGS. 4 and 5 are drawings for explaining a motor driver of prior art.
FIG. 4 is a circuit block diagram of a three-phase DC motor driver of the prior art. Reference numeral 1 denotes a three-phase DC motor and numeral 2 designates a motor driver for supplying electric current to a winding U phase 3, a winding V phase 4 and a winding W phase 5 of the three-phase DC motor 1. An FG pattern 6 is adapted to output a signal of a frequency proportional to a number of revolutions of the three-phase DC motor 1. In an FG amplifier 7 for converting signals to pulse signals, the signal output from the FG pattern 6 is wave-shaped to be converted to an FG signals 8. The FG signal 8 is input into a speed discriminator 9 for controlling the number of revolutions of the motor, in which the FG signal 8 is compared with a previously provided reference FG period to output an acceleration signal 12 or a deceleration signal 13 so that the number of revolutions of the motor may become a set number of revolutions. Reference numeral 10 designates a crystal oscillator generating a reference clock for the speed discriminator 9. The reference FG period is sent to the speed discriminator 9 by a reference FG period signal 70. Reference numeral 19 denotes an ON/OF signal for starting or stopping the DC motor.
A charge pump 14 serves to charge and discharge electric current into and from a charge pump capacitor 15 and a charge pump capacitor 16 in accordance with the acceleration signal 12 and the deceleration signal 13 so that an error in number of revolutions is converted to DC voltage. A resistance 17 will adjust phases of return amount from the DC motor. A torque amplifier 20 amplifies the difference between the DC voltage and a reference voltage 21 to output a signal to a current limiting comparator 18 that detects excess current when overloading. A current limiting resistance 51 converts current value of the DC motor 1 to voltage that is detected at the inversion terminal of the current limiting comparator 18, and if the detected value is more than a reference voltage 52, the current is cut off. In other words, if excess current is applied to the DC motor 1, the current is cut off so that the current becomes less than a set current value. With the exception of being overloading, the output from the torque amplifier 20 is directly output into a hall amplifier 22 and a PWM generator 23.
In accordance with a DC voltage level of the output of the torque amplifier 20, the hall amplifier 22 amplifies outputs from a hall element U phase 24, a hall element V phase 25 and a hall element W phase 26 to output the amplified signals into a PWM comparator U phase 27, a PWM comparator V phase 28 and a PWM comparator W phase 29. The hall elements 24, 25 and 26 are supplied with current from a 24 voltage power source 30 through hall element biasing resistances 31 and 32 to output positional information of the rotor as voltage waveforms.
The PWM generator or PWM drive circuit 23 produces a PWM signal 33 as a reference for switching drive of the DC motor 1. The frequency of the PWM signal 33 is set by a PWM frequency setting resistance 34 and a PWM frequency setting capacitor 35.
Outputs of the hall amplifier 22 and the PWM drive circuit 23 are sent to the PWM comparators 27, 28 and 29 for the respective phases. The PWM comparators 27, 28 and 29 compare the output of the hall amplifier 22 with the output of the PWM drive circuit 23. If the output of the hall amplifier 22 is more than that of the PWM drive circuit 23, the comparators output an H level signal to supply current (or power) to the motor. Reversely, if the output of the former less than that of the latter, the comparators output an L level signal to cut off the supply of the current (or power). In other words, an ON_duty ratio for the switching drive of the DC motor 1 is determined.
Reference numeral 85 designates a printer driver for driving an upper FET_U phase 36, an upper FET_V phase 37 and an upper FET_W phase 38 and a lower FET_U phase 39, a lower FET_V phase 40 and a lower FET_W phase 41 in accordance with the outputs of the PWM comparators 27, 28 and 29. A Zener diode U phase 42, a Zener diode V phase 43 and a Zener diode W phase 44 protect gate to source connections from voltage when the respective phases are at high impedance.
Reference numeral 45 denotes a booster circuit for switching the upper transistors (36, 37 and 38) for the respective phases. A voltage waveform output from a boosting oscillator 46 is bypassed to the next step by a by-pass capacitor 47 so that the voltage waveform is rectified by a rectifier diode 48, biased to the power-supply voltage by a DC bias diode 49 and smoothed by a booster capacitor 50.
FIG. 5 is a time chart for explaining a principle for supplying sinewave current to the motor windings (3, 4 and 5) to control the supply capability to the motor depending upon loads being applied. Reference numerals 55, 56 and 57 designate artificial or pseudo sinewaves obtained by amplifying the amplitudes of output voltages of the hall element U phase 24, hall element V phase 25 and hall element W phase 26 by an amplification factor proportional to the output voltage of the charge pump 14 at the respective phases. Reference numeral 58 denotes a triangular wave produced by the PWM drive circuit 23. The PWM comparators 27, 28 and 29 compare the artificial sinewaves 55, 56 and 57 with the PWM triangular wave 58 to produce coil applying voltage waveforms 59, 60 and 61 at the respective phases, thereby applying voltages to the windings of the respective phases. Numerals 62, 63 and 64 indicate winding currents to be supplied to the windings 3, 4 and 5 of the DC motor 1 by the coil applying voltages.
In the motor driver of the prior art, however, the following problems remain to be solved.
According to the characteristics of a usual DC motor, when voltage is applied to its windings, current flowing through the windings is only progressively increasing under the influence of inductance value of the windings and induced voltage of the motor. In other words, the current flowing through the windings tends to rise behind the voltage applied to the windings. When the current lags behind the voltage, the winding current 72 also lags in phase behind the winding voltage 71 by a time as shown at 74 in FIG. 6. Due to the delay in phase, therefore, the current flowing direction (or the power supplying direction) of the windings may not be completely switched during the switching of magnetic poles of the rotor magnets so that there is a time in which current (or power) is supplied to apply a force in a direction opposite to the rotating direction of the rotor. This phenomenon will be referred to herein as “braking action 73”.
In order to solve this problem, it has been proposed that hall elements for detecting the position of a rotor are mounted in the rotor to be advanced relative to a stator (referred to hereinafter as “shift mounting”) as shown in FIG. 7 wherein the shifted amount is shown at 75. In this manner, the switching of the phases is effected earlier than the switching of the magnetic poles inherently effected by the rotor so that the switching point 76 for switching the current flowing in and out of the windings may become in coincidence with the point for switching the magnetic poles inherently effected by the rotor. According to the “shift mounting” of the hall elements, no braking action occurs so that the maximum motor efficiency may be achieved. In this case, however, when the motor is about to be started, the induced voltage is not yet generated because the motor is under inoperative condition. Namely, as shown in FIG. 8 there is little or no phase difference between the winding applying voltage and the winding current in contrast with the case of normal rotation of the motor. When the hall elements are shift-mounted under no phase difference condition, the braking action would occur as shown at 77 in FIG. 8, leading to reduced starting torque which is a further problem.